INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Soil classification: a proposal for a structural approach, with reference to existing European and international experience Classification des sols: une proposition pour une approche structurelle, tenant compte de l’expérience Européenne et internationale lr. Gauthier Van Alboom - Geotechnics Division, Ministry of Flanders, Belgium ABSTRACT: Most soil classification schemes, used in Europe and all over the world, are of the basic type and are mainly based upon particle size distribution and Atterberg limits. Degree of harmonisation is however moderate as the classification systems are elabo­ rated and / or adapted for typical soils related to the country or region considered. Proposals for international standardisation have not yet resulted in ready for use practical classification tools. In this paper a proposal for structural approach to soil classification is given, and a basic soil classification system is elaborated. RESUME: La plupart des méthodes de classification des sols, en Europe et dans le monde entier, sont du type de base et font appel à la distribution des particules et aux limites Atterberg. Le degré d’harmonisation est cependant modéré, les systèmes de classification étant élaborés et / ou adaptés aux sols qui sont typiques pour le pays ou la région considérée. Les propositions pour normalisation internationale n’ont à ce point pas encore résulté dans des outils pratiques de classification. Cette note propose une approche structurelle pour la classification et un système de classification de base est élaboré. 1 INTRODUCTION have an extended soil classification which goes beyond the clas­ sical approach and also takes into account geotechnical proper­ 1.1 General versus regional soil classification ties of the soil. Extended soil classification includes, density, Soil classification seems to be a topic where it is hard to find the watercontent, strength properties, ... and needs undisturbed greatest common denominator of existing national systems. samples for testing. Most classification systems are however based upon particle size In the author’s opinion the added value of an extended soil distribution and Atterberg limits, as basic tools. classification is not obvious, as appreciation of geotechnical It might not prove difficult to develop a general classification characteristics such as shear strength is difficult to put in general scheme, which is apt to conceal the different criteria put forward rules, and should be left to engineering judgement. However in for “standard” soiltypes (sand, clay, silt, ...). It is however not the proposal multi-layered system the extended soil classifica­ possible to differentiate a general classification scheme for par­ tion system can be fully incorporated. ticular differences in soils occuring in countries all over the world (e.g. coarse silt in Spain, loam in Belgium, France). In­ 1.4 Soil classification from field tests stead of creating a system that intends to include most of these particularities, or just bluntly denying their existence, one should Soil description out of results of field tests (mainly CPT) is also create a multi-layered system. This system would consist of a a much covered item and one might consider it useful to incor­ basic international soil classification system, with a general out­ porate soil description based upon field testing as an item in the line of basic soil classification and an additional optional re­ classification system. gional classification system. 2 MULTI-LAYERED SOIL CLASSIFICATION SYSTEM 1.2 Soil classification versus soil description A multi-layered soil classification system could be subdivided in Some classification schemes (ASTM, BS) also include visual following sections description (for organic soils). In the author’s opinion one should not mix a purely objective scheme based upon laboratory test results with the more subjec­ 2.1 Basic soil classification (BSC) tive appreciation by visual description. That does not mean that visual description should be simply put aside, but instead of Basic soil classification gives a first assessment of the nature of mixing up with the laboratory test results based classification the soil considered, and does not need undisturbed samples. It visual description would form one more layer of the global sys­ consists of two different parts, which are complementary, but tem. As visual soil description is complementary to soil classifi­ may stand on their own: cation on a basic level, it could be put under the heading of basic a. visual soil description (VSD) soil classification. b. basic soil classification scheme (BSCS) with a pure classi- ficational approach, and based on the results of following soil classification tests: particle size distribution, Atterberg 1.3 Basic soil classification versus extended soil classification limits, organic content and calcareous content. Soil classification can be performed on different levels and the ongoing trend is to incorporate more and more geotechnical pa­ 2.2 Extended soil classification rameters. Therefore it’s necessary to first indicate the different levels of classification to be considered. Soil classification based on results of soil classification tests and On the one hand we have a basic soil classification, as in determination of more specific geotechnical parameters (density most existing soil classification systems, on the other hand we index, water content, strength properties, ...). 527 2.3 Regional soil classification and coefficient of curvature Optional addendum to the basic and extended soil classification; d 3Q2 Cc (Cc ) (2) relates to national soil classification procedures in order to give D10 - ° 6 0 information on typical aspects for classification of soils which If however D!0 is less than 2 urn the coefficient of uniformity might not be included in general soil classification. and of curvature will not be calculated, and no evaluation of the grading of the particle size distribution will be given: 2.4 Soil classification from field tests gravel - well graded Cu > 4 and 1 < Cc < 3 Optional addendum to the above mentioned classification sys­ - poorly graded Cu<4 and/or Cc<l,C c>3 tems. Soil classification form field tests is not based on descrip­ sand tion or testing of samples, but emanates from interpretation of - well graded Cu > 6 and 1 < Cc < 3 field test results. Well known is the soil classification from CPT- - poorly graded Cu < 6 and/or Cc < 1, Cc > 3 testing, using e.g. charts developed by Robertson et al. 3.2.2 Atterberg limits 3.2.2.1 Plasticity chart 3 PROPOSAL BASIC SOIL CLASSIFICATION SCHEME The division of fine soils is made on basis of the relation be­ 3.1 Introduction tween liquid limit and plasticity index, as proposed by Ca- sagrande. On the extended plasticity chart the A-line represents After a study of different European classification systems (from the borderline between clays and silts. UK, Germany, France, Belgium, Netherlands) and the ASTM A soil is named clay when Ip is not less than 7. In the plasti­ classification system a basic soil classification scheme (BSCS) city chart the soil plots on or above the A-line. A soil is named was developed. silt when Ip is less than 0,73 (wL - 20) and / or less than 4. In the The BSCS is based on particle size distribution, Atterberg plasticity chart the soil plots under the A-line. limits (including plasticity chart) and organic and calcareous In the hatched area - for Ip values between 4 and 7 - the soil matter. is given a double name, clay or silt. 3.2 Parameters o f soil classification 3.2.2.2 Liquid limit Degree of plasticity of fine grained soils is given in function of 3.2.1 Particle size distribution liquid limit value: - low plasticity wL < 35% 3.2.1.1 Subdivision in particle size categories - intermediate plasticity 35%< wL < 50% For classification purposes 3 categories of soils are considered: - high plasticity 50% < wL < 70% - coarse grained soils less than 10% particles < 60 |im - very high plasticity 70% < wL < 90% - fine grained soils more than 40% particles < 60 |im - extremely high plasticity wL > 90% - mixed grained soils between 10% and 40% particles < 60 nm For coarse grained soils only particle size distribution is consi­ 3.2.3 Organic content dered in the soil classification scheme. For fine and mixed Determination of organic content is not harmonised and different grained soils both particle size distribution and Atterberg limits methods exist, resulting in a wide scatter of adopted limiting are considered. values. Moreover the appreciation of organic content in different Note: the value of 60 |im can be replaced by the corresponding countries is relative to the maximum values of organic content value of 63 |im if only sieving was performed for particle size encountered in those regions. The French practice to distinct 4 distribution. classes, with addition of a fifth class for peat soils is adopted: - inorganic OC < 3% 3.2.1.2 Main particle size fractions and subdivisions - slightly organic 3% < OC < 10% The main particle size fractions and subdivisions (based on the - moderately organic 10%<0c<30% internationally accepted 2,6 system) are as follows: - highly organic 30% < OC < 50% - peat OC > 50% Table 1. Main particle size fractions. 3.2.4 Calcareous content The remark formulated in § 3.2.3 for determination and appre­ soil fraction qualifying term particle size (mm) ciation of organic content is also valid for calcareous content. - boulders d > 200 It should also be noted that in existing classification schemes cobbles - 60 < d < 200 criteria for calcareous content result in a denomination with a coarse 20 < d < 60 more descriptive background (limestone, marl), rather than a gravel medium 6 < d < 20 comparative denomination (slightly, moderately).